Typically the horsepower we measure in cars is the engine power. The angular velocity in that equation is the engine RPM, and the power is the rate of work (force over distance) that engine is capable of outputting at a given RPM. Because it's based on the engine RPM, this doesn't depend on the gear ratio.
When we talk about torque, it can be measured either at the crank (engine torque) or wheels (wheel torque). Wheel torque represents the amount of force applied by the wheel to the road, and depends on the gear - a lower gear will output more torque at a given RPM than a higher one. This will result in greater acceleration.
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